FAQ1103

Applies To These Products

Question

Is it possible to apply a negative voltage to the Analog Input (pin 5) on the MPT Series Controllers?

Answer

No, the Analog Input on the MPT Series Temperature Controllers is designed to accept a positive remote setpoint voltage in the range from 0 to +5 V, with the optimum range of operation for stability from +0.25 to +2.5 V. Additionally, the Analog Input voltage should always remain below the supply voltage or damage to the controller may occur.

The final temperature setpoint voltage is monitored via the Temp. Setpoint Monitor (pin 11). There is a transfer function associated with the Analog Input of 0.5 ⋅ VIN that will sum with any voltage created by the on-board Temp Setpoint potentiometer to achieve the final setpoint temperature voltage.

As an example, say we are using a 10 k&ohm; thermistor and wanted to temperature control a system at 29°C. The thermistor bias current (IBIAS) from the controller is set to 100 µA. At 29°C the thermistor will exhibit a resistance (RTHERM) of about 8408 &ohm;. So the expected voltage at 29°C across the thermistor would be 0.840 V:

IBIAS ⋅ RTHERM = VSETPOINT
⇒ 100 µA ⋅ 8408 &ohm; = 0.8408 V.

We wish to control this setpoint voltage completely through the remote voltage input via the Analog Input so the on-board potentiometer is adjusted to zero Volts (at least 12 full turns counterclockwise). Because of the transfer function, we apply 1.68 V to the Analog Input (pin 5) and monitor a final setpoint voltage of 0.840 V on pin 11 (Temp. Setpoint Monitor).